The present disclosure relates to an image forming apparatus, such as a copier or a printer, provided with a fixing device which fixes a toner image having been transferred to a recording medium. More particularly, the present disclosure relates to a method for estimating the lifetime of a fixing member such as a fixing roller or a pressing roller.
In conventional image forming apparatuses utilizing electrophotography, an image forming process proceeds as follows. An image carrying member such as a photosensitive drum having been electrostatically charged uniformly by a charging device is irradiated with laser light from an exposing device to from a predetermined electrostatic latent image with partially attenuated electrostatic charge, and toner is attached to the electrostatic latent image by a developing device to form a toner image. Then, the toner image is transferred to a sheet (recording medium) with a transferring means, and unfixed toner is heated and pressed by a fixing device to form a permanent image.
The fixing device is a device which, while conveying a sheet, melts toner with a fixing member composed of a heated rotary member, such as a fixing roller or a fixing belt, and a pressing member, such as a pressing roller. The worn state of the surface of the fixing member changes depending on the driving time and the number of sheets passed. Continuous use of the fixing member after it has reached the end of its useful life causes image defects and fixing failure.
Thus, to maintain stable image quality over a long period of time, it is necessary to accurately sense the surface condition of the fixing member including a heated rotary member and a pressing member, and to accurately estimate its lifetime. Specifically, since the degree of wear varies in the axial direction, for example, with the size of sheets fed, it is necessary to accurately sense the surface condition of the heated member and the pressing roller.
Accordingly, various methods have been proposed for estimating the lifetime of the fixing device; for example, an image forming apparatus is known which includes a temperature sensing means for sensing the temperature of the fixing device, a counting means for counting the time taken for a recording material of a given size to pass through the fixing device, and a control means for judging the lifetime of the fixing member based on the temperature sensed by the temperature sensing means and the time counted by the counting means.
Also an image forming apparatus is known in which the number of times that it has started up from a state equal to or lower than a predetermined temperature is recorded, and according to the number of times, the condition of the fixing device is estimated, and thereby the temperature of the fixing device is controlled so as to reduce wear of the fixing device while reducing power consumption.